Display device based on bistable electrostatic shutter

ABSTRACT

Electromechanical shutter and display comprising a two-dimensional matrix of such shutters are proposed, in which the membrane under the force of electrostatic attraction moves from its original position parallel to the substrate plane into a final position normal to the substrate plane, thereby transferring the shutter, or display pixel, from its “off” state into its “on” state. To produce the “on” state, the electrostatic force is applied only to a narrow conductive strip placed on the side of the membrane and rotates the membrane around the torsion hinges attached to this strip. Two-and three-electrode shutter and pixel configurations are considered. Both versions provide a bi-stable membrane behavior, which implies that the voltage needed to transfer the membrane into the “on” state is larger than the voltage needed to maintain the membrane in this state. This feature of bi-stability allows realization of functions “pixel hold” and “pixel select” using a simple passive matrix architecture. In the “pixel hold” state, after forming image on the screen, the display consumes essentially no power. The “on”-to-“off” contrast ratio is expected to be high. These properties are very attractive for utilization of this display for electronic book application.

FIELD OF THE INVENTION

The present invention relates to the bi-stable electrostatic opticalshutter, and more particularly, to the flat panel display containing atwo-dimensional array of these shutters.

BACKGROUND OF THE INVENTION

The electrostatic optical modulator employing a resilient electrodemoving over a static electrode has been a subject of multiple patentsand publications. The difference between these modulators originatesessentially from the shape of electrodes involved as well as thedirection and spatial limitations of movement of the flexible electroderelative to the static electrode, see e.g. U.S. Pat. Nos. 4,229,075;4,208,103 and 4,786,149. Lateral membrane movement is used in opticalshutter and display of U.S. Pat. No. 6,288,824, wherein theelectrostatically moved membrane and static electrode consist ofperiodic metal stripes, so that lateral membrane movement opens orcloses multiple slits to pass or shut the light. Another approach, seeU.S. Pat. No. 6,600,474, relies on a flipping of the membrane by 180degrees, and latching it at this position by an electrostatic force. Noresilient force is applied, since the membrane freely turns around thehinges between two extreme positions.

Various examples of the electrostatic optical modulators are based onthe field induced bending of a cantilever membrane which is moved towarda flat static electrode, thereby changing the optical state, see U.S.Pat. Nos. 3,553,364; 3,600,798; 4,229,732; 4,731,670 and 5,781,331.Another approach relies on an electrostatically induced twisting of themembrane mirror from its initial flat position to vary the opticalreflection, see U.S. Pat. Nos. 3,746,911 and 4,710,732. Thiselectrostatic optical modulator, known as Deformable Mirror Device(DMD), is currently commercially used in a projection display.

The DMD pixels represent a densely packed mirror array reflecting lightbeams into the objective lens when the pixels are unbiased (“on state”)and moving the reflected light out of the objective lens when they areelectrostatically tilted ˜10 degrees (“off state”). The limitations ofapplication of the DMD strictly to projection display originates fromthe small tilt of the twisted membrane, since 10 degree rotation is notenough to use the DMD pixels for the flat panel displays.

In the proposed shutter and display, according to the present invention,this limitation is lifted due to different pixel design which allows formembrane tilt of 90 degrees, thereby making it suitable for applicationto the flat panel displays.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to introduce newelectromechanical optical shutter based on a large membrane rotationunder the electric field.

Another object of the present invention is to introduce a new flat paneldisplay in which these shutters compose an electrically controlledtwo-dimensional array of pixels.

Yet another object of the present invention is to provide the detaileddescription of the operation, fabrication and performance of theproposed flat panel display.

According to the present invention, the shutter comprises a membranedisposed over and separated from the substrate by an air gap. Themembrane is held parallel to the substrate by four pillars grown on thesubstrate. Two pillars support the membrane through conductive hingesattached to the membrane, while two other pillars support the membraneat its opposite side. The pillar height controls the air gap space.

The membrane consists of several layers:

-   -   the bottom conductive layer connected to the hinges;    -   the insulator above the bottom conductive layer;    -   and the top layer having high optical reflectivity in one        shutter embodiment and black light absorbing surface in another,        see below.

The top layer is isolated form the bottom conductive layer on themembrane and does not participate in the process of electricalactivation of the shutter. The bottom conductive layer, made as aconductive stripe, about 3–4 μm wide, and connected to the membranesupporting hinges, occupies a small portion of the membrane and servesas one of the electrodes for electric field induced membrane movement.This layer is shifted to on one of the sides of the membrane and thuspositions the membrane asymmetrically relative to the membranesupporting hinges.

The second controlling electrode is made in the shape of a narrow metalstripe and placed on the substrate underneath and parallel to the bottomconductive layer of the membrane. When a potential difference is appliedbetween the bottom conductive layer on the membrane and the secondelectrode on the substrate, the electrostatic force rotates the membraneto reduce the distance between the active electrodes. If the air gap isdeep enough to accommodate the width of the bottom conducting layer, thetwisted membrane can reach the position normal to the substrate plane,and thereby fully open the shutter to pass the light or drasticallychange the shutter reflection. Thus, unlike the previously discussedDMD, having small rotation angle for the membrane, the proposed shutterallows large shutter opening and therefore can be used for fabricationof the flat panel display.

It is important that membrane final position relative to the substratecan be maintained by a controlling voltage which is lower than thevoltage needed to turn the membrane into its upright position. Such abi-stability effect exists due to increase of the capacitance betweenthe active electrodes, as the membrane electrode approaches theelectrode on the substrate. As discussed below, in the process ofmembrane lifting from its original position, the membrane enters theregion of instability and snaps into the final state corresponding tothe fully opened shutter. This state can be maintained indefinitely longby a relatively small voltage in comparison with the original voltagerequired to transfer the membrane into this fully opened position.

A two-dimensional array of these shutters, with the appropriate networkof connecting lines, forms a display matrix, wherein each shutter withassociated connection lines forms an elemental display pixel. Asdiscussed below, the bi-stability feature is helpful to simplify thedisplay driving scheme. The proposed mechanism of optical modulationresults in two different display implementation, namely, as a lightreflective display, or as a light transmissive display. In the lattercase, a back light is needed to create an image on the screen.

The display fabrication relies on the well knownMicro-Electro-Mechanical System (MEMS) technology, when the membranes orcantilevers are made with selective etching of sacrificial layersdeposited during a multi-layer fabrication process. The flexible filmsof metals and insulators (or their combination) remaining in thestructure after removal of the sacrificial layers, are known to possesextreme mechanical strength, flexibility and long life time ofoperation.

The primary use for the proposed display would be the electronic book,when the text remains on the screen for a long time. There are severaladvantages for this particular application:

-   -   1. The display practically consumes no power, since there is no        current in the pixels, an important condition for the e-book        operation. This implies that only reflective display mode will        be useful for this application.    -   2. Once the image appears on the screen, typically as a black        text on a white background, it is then maintained indefinitely        by constant voltages applied to the columns and rows. No image        scanning, gray levels or moving picture are required in this        case. This implies that during the exposure of a page on the        screen, each membrane will be locked at its extreme fully “on”        of fully “off” positions until the page is changed.    -   3. Due to the bi-stability effect, pixel selection does not        require an active matrix network and can be done in a passive        mode through direct activation of appropriate rows and columns.    -   4. The useful area in the pixel is large, and the fill factor        can be made within 95%. This implies a good white color of the        electronic “paper”, if the pixel in its “off” state reflects the        light from the top layer. High fill factor also indicates a good        black color for the image, when the membrane is twisted to its        “on”-state position. The exposed area underneath the membrane is        coated with the black powder to maximize the on/off contrast        ratio.    -   5. Since the image on the screen is maintained for a long time,        there is a little wear of the twisted parts, which warrants a        long life time.

The pixel design of the proposed display is simple, and its fabricationis well within planar, MEMS-based technology. Only four photolithographymasks are required for the fabrication (see below). This implies lowcost for the device production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Top view of the electromechanical shutter.

FIG. 2. Cross-sectional view of the shutter in:

-   -   a. unbiased state (“off”-state)    -   b. electrically biased state (“on”-state).

FIG. 3. Structure of the active electrode on the substrate fortwo-electrode pixel design.

FIG. 4. Illustration of forces applied to the membrane.

FIG. 5. Top view of four pixels of the display.

FIG. 6. Matrix of pixels for two-electrode pixel structure.

FIG. 7. a. Structure of the active electrode on the substrate forthree-electrode pixel design.

-   -   b. Matrix of pixels for three-electrode pixel design.

FIG. 8. Processing steps for display fabrication.

DETAILED DESCRIPTION

FIG. 1 shows the top view of the shutter 10. The square shape membrane11 rests on four pillars 12, 12′, 15 and 15′ grown on the substrate (notshown). Two stripes 13, made by cutting notches 14 in the membrane 11,attach the membrane to the pillars 12 and 12′ and provide electricalconnections to the external voltage source. At the same time, thestripes 13 serve as torsion hinges for membrane rotation around the axisOO under the electrostatic forces (see below).

The torsion hinges are shifted to the left side of the membrane, ˜3–4 μmaway from the left edge of the membrane. Therefore, the membranerotation lifts the largest, right portion of the membrane, while a smallmembrane area on the left side from the hinges will be depressed towardthe substrate. The stripes 13 have an appropriate length to provide aresilient force for the membrane twist around the hinges: they should belong enough to minimize the electrostatic force and therefore, externalvoltage for the membrane rotation, yet sufficiently strong for quickmembrane return to its original flat position when the voltage isswitched off.

Depending on the shutter design and application, there are severaloptions for coating the top membrane surface. In the case, when theshutter is used as a light reflective modulator, it is preferable tocoat the top surface with light reflecting material, such as layer ofAl, or white paint or powder, while the deeper part of the shutterunderneath the membrane (not shown) is blackened to increase the on-offcontrast ratio. The alternative embodiment would be coating top membranesurface with a black paint/powder and coating the substrate areaunderneath the membrane with white paint/powder. In the first case, theunbiased, “on”-state, of the shutter has white color, while in thesecond case it has black color. In another, transmissive, mode ofoperation, when the shutter modulates the back light passing through thedevice, it is preferable to blacken the top membrane surface.

The active membrane area, participating in the light modulation is largeand provides the shutter fill factor close to one. Only the notches 14and partially the stripes13 do not contribute to the light modulation.

FIG. 2 shows a cross-sectional view of the shutter 20 in unbiased (a)and biased (b) state. The membrane 21 consists of three layers: thebottom conductive layer 22, the insulating layer 23 and the top layer24. The latter, depending on the shutter design, can be made as lightreflecting layer or black color film. In the case, when the shutteroperates in the light reflecting mode, it is preferable to make the toplayer as a white color reflecting film, and a highly reflective metal,such as Al, is the most appropriate material for this layer.

The top film 24 is not electrically connected to the membrane drivingcircuit, while small, 3–4 μm wide, stripe 22 of the bottom conductivelayer serves as one of the electrodes for the membrane activation. Thisstripe extends to the pillars 25 to which the power supply 28 isattached.

The second electrode of the driving circuit is deposited as a narrowmetal stripe 26 on the substrate 27. This stripe extends parallel to thestripe 22 of the membrane 21 (i.e. normal to the plane of the drawing)in close proximity to the pillars 25 and is connected to the powersupply 28. The detailed view of the layer 26 will be shown later inreference to FIG. 3.

When the power supply 28 is disconnected, FIG. 2 a, the membrane 21rests on the pillars 25 and thus constitutes “off”-state of the shutter.As the voltage is applied between the electrodes 22 and 26, theelectrodes acquire an electrostatic force of attraction. At anappropriate bias, electrode 22 on the membrane starts moving toward theelectrode 26 on the substrate, thereby lifting up the right (largest)part of the membrane. The height of the pillars 25 is made slightlyhigher than the width of the strip 22, so that the membrane can move toits final, “on-”state position of the shutter, which corresponds to themembrane plane normal to the substrate plane, FIG. 2 b. The shutter isthen fully open, and capable of transmitting light, if the substrate 27is made from a transparent material, such as glass, while the topmembrane surface is coated with a black powder to enhance the“on”-to-“of” contrast ratio. On the other hand, when the reflective modeof the shutter operation is chosen, the substrate underneath themembrane is blackened, so that in the “on”-state the shutter absorbs thelight, while in the “off”-state the metal on top of the membranereflects the light. Another possible embodiment of the reflectivedisplay includes coating the top membrane surface with a white paint orpowder. No metal deposition on top of the membrane is needed in thiscase. The third version of the reflective display, can be implemented bymaking the “off”-state black by depositing black powder on top of thedisplay, while the “on”-state corresponds to the white color. White orlight reflecting dielectric paint or powder deposited on the substrateprior to the membrane fabrication is preferred method of making whitecolor. It is important that the reflective mode is best suitable for thee-book applications, since no backlight is required for operation inthis case, and therefore, no power is consumed.

FIG. 3 shows the top view of the shutter underneath the membrane. Fourpillars grown on the substrate support the shutter. The first twopillars, 31 and 31′, serve for both the support of the membrane and theelectrical connection of the active electrode 33 through torsion hinges36 located on the membrane (dashed line). Two other pillars, 32 and 32′,are used only for the membrane support.

The metal stripe 34 represents the second active electrode of thedriving circuit. It is placed on the substrate parallel to andunderneath the active electrode 33 on the membrane. The electricalaccess to this electrode is accomplished by connecting it to thehorizontal conductive line 35. This line serves as a row line in thedisplay matrix, while connections of the membrane electrode 33 throughthe torsion hinges 36 and pillars 31 and 31′ form a vertical, columnline of the matrix network, see below. To avoid direct electricalcontact of the active electrodes 33 and 34 when the membrane is in thevertical position (“on”-state), the electrode 34 and the conducting line35 are coated with an insulating film of thickness t. In thecalculations below, this film is assumed to be the smallest distance dbetween the active electrodes, d=t, when the membrane is moved to itsupright position.

At some moment of the membrane movement under the electrostatic force,the membrane enters the region of instability. The instability is causedby super-linear increase of the force of electrostatic attraction anddecrease of the membrane weight as the membrane moves toward its final,vertical position, see below. This feature results in the membranebi-stability, implying that fully “on”-state can be supported by avoltage, lower than the voltage needed to move the membrane into thisstate.

The forces applied to the membrane 40 are shown in FIG. 4. Three forcesare taken into account: the electrostatic force of electrode attraction,the membrane weight and the resilient force of membrane hinges. For thesake of estimates, all formulas were simplified and used only fordemonstration of the effect of bi-stability. The electrostatic forceF_(e) lifting the membrane isF _(e) =SV ²/2d ²,where S is the area of the stripe 41, V the voltage applied, and d thedistance between the active electrodes 41 and 42. Distance d varies fromd≈l (l is the height of the pillar 43), when the membrane lies on thepillars, to a very small value, determined essentially by the insulationthickness t between the active electrodes 41 and 42, d=t, when themembrane is twisted to its extreme position normal to the substrateplane.

The membrane resilient force isF _(r) =γ·α,where γ is the elasticity coefficient of the torsion belts and α is theangle of the membrane tilt from its original state. This force dependson the design and geometry of the torsion hinges. One can change thelength and width of the supporting belts, their thickness andfabrication materials, and thus vary the coefficient γ in a wide rangeto adjust the resilient force for obtaining the effect of bi-stability.

The membrane weight is P_(m)=mg, where m is the membrane mass. The forceacting on the membrane is the weight component in the direction normalto the membrane plane P_(mn), which is tilted by the angle α from itsoriginal position, isP _(mn) =P _(m)·cos α=P _(mn) ·d/l.

In the equilibrium, the torques (momenta of forces) acting on themembrane lever are equal:(F _(e) −F _(r))·l=P _(mn) ·Lwhere L is the membrane length. As the membrane moves up, the distance ddecreases, and all three forces begin to change: F_(e) increasessuper-linearly ˜1/d², P_(mn) decreases linearly ˜d and F_(r) increasesproportionally to angle α. At some tilting angle α, the super-linearincrease of F_(e), and the decrease of the weight component P_(mn),cannot be compensated by the linear increase of the resilient forceF_(r), and the system snaps into final extreme position at d=t andα=π/2.

One can estimate the voltages needed to keep the membrane in the abovediscussed two extreme positions. In the original, “off”-state, d=1 andα=0. In this case, equating the torques applied to the membrane leverfrom the left side (electrostatic force) and from the right side(membrane weight) one obtains:1·F _(e)=(L−1)·mg,where L is the total membrane length. This equation defines the voltageV_(lift) needed for lifting the membrane. At reasonable values of L=200μm, 1=4 μm, and for SiO₂-based, 0.5 μm-thick membrane, one obtainsV_(lift)˜1V.

In the final, vertical, membrane position, P_(mn)=0, d=t, andF _(e) =SV ²/2t ²=γ·π/2.

In this case, minimum voltage needed to hold the membrane in thevertical position, V=V_(on)=(γπt²/S)^(1/2). Since t<<1, at a reasonablevalue of γ one can easily obtain V_(on) much less than the voltageV_(snap) needed to transfer the membrane into the final “on”-state.Therefore, the “on”-state of the shutter can be maintained by a voltage,significantly lower than the voltage needed to transfer the membraneinto this state. This implies that after application of a relativelyhigh pulsed voltage V=V_(snap) and transferring the membrane into thefinal “on”-state, this state can be supported indefinitely by a smallconstant voltage V_(on). At the same time, the voltage V_(on) willpractically not affect the pixel in the “off”-state, since theelectrostatic force needed to produce the “on”-state F_(e,snap)˜V_(snap)² is much greater than the supporting force F_(e,on)˜V_(on) ². This isimportant factor for simplifying the driving architecture of thedisplay, composed of a two-dimensional matrix of these shutters, seebelow.

The display, according to the present invention, consists of atwo-dimensional array of the above described shutters, wherein eachshutter, together with the associated electrical connections, representsa pixel element. To control the pixels in the display matrix and thuscreate an image on the screen, the pixels in the matrix must beappropriately connected, and column and row voltages are to be applied.

One of the exemplary embodiments of the display matrix is illustrated inFIG. 5. Four pixels of the matrix 50 in the “off”-state are shown. Themembranes 51 rest on the pillars 52. The membrane active electrodes (notshown) located at the membrane bottom, as shown in FIG. 2 and FIG. 3,are connected through their torsion hinges along axis OO and thus formcolumn lines of the display. All substrate active electrodes 53, shownwith dashed line, are connected in each horizontal pixel array to themetal line 54, and thus form row lines of the display. The intersectionof row and column line identifies the pixel location.

The pixel is transferred into “on”-state as the associated columnvoltage exceeds the switching voltage V_(snap), while the associated rowline is grounded. After switching the pixel into the “on”-state, thisstate can be maintained by lower column voltage V=V_(on). Line-by-linesequential enabling of the rows with appropriate activation of thecolumns, creates a “still” image on the screen. The image will bemaintained as long as the potential difference between the column androw lines remains V_(on).

FIG. 6 illustrates exemplary approach in the display charging procedure,allowing one to obtain the above mentioned “still” image, suitable forthe e-book application. Nine pixels in the matrix are shown as squareboxes connected to the row and column lines. In the original “off”state, all rows have the voltage ½V_(snap) while all the columns havethe voltage ½V_(snap)+V_(on), so that the resultant voltage drop acrosseach pixel is V_(on). Pixel 5 is charged by applying additional datapulse of amplitude ½V_(snap) to the middle column line and simultaneousenabling the middle row line by grounding it. The resultant voltageacross this pixel becomes V_(snap)+V_(on), which is sufficient to switchthe pixel into the “on”-state. All other pixels in the 9 pixel matrixhave the voltage drop V_(on) (across pixels 1,3,7 and 9) or½V_(snap)+V_(on)<V_(snap) (across pixels 2,4,6 and 8) and therefore willremain in their “off”-state. After sequential enabling of all rows inthe display and thus creating the image, all pixels obtain theiroriginal voltage drop of V_(on), which will keep the image on the screenindefinitely long.

The above discussed two-electrode pixel driving scheme may have someproblem for its realization in a real device. More detailed analysis ofthe pixel matrix of FIG. 6 shows presence of multiple pixel connectionsacross the entire network. Al pixels are connected to each other throughadditional circuit loops, having at least four pixels in each loop.Dotted lines in FIG. 6 show two examples of such loops: pixels 4,5,7 and8 form the first loop, while pixels 2,3,5 and 6 form the second loop,with pixel 5 involved in both of them. This implies a cross-talk betweenthe pixels and additional voltage drops across the pixels in the processof initial pixel charging to create an image on the screen. As a result,during the pulsed charging process of the pixel 5, as shown in FIG. 6,pixels 4 and 6 acquire additional voltage equal to ⅓ of the pulsedvoltage ½V_(snap), thereby yielding the total voltage on these twopixels of ½V_(snap)+⅙V_(snap)+V_(on)=⅔V_(snap)+V_(on). Other six pixelsin the matrix acquire at this moment a total voltage drop of⅙V_(snap)+V_(on). Although the voltage on the pixels 4 and 6 of⅔V_(snap)+V_(on) is lower than V_(snap) required for switching the pixelinto the “on”-state, such a small difference between these two voltageamplitudes may cause a pixel malfunctioning, when pixel properties arenot absolutely identical, and pixel-to-pixel variations of V_(snap) mayoccur, for instance, due to spatial variations of the resilient force.

To avoid complications related to the above discussed cross talkanother, three-electrode, pixel design can be employed. In this case,any additional potential drops across the pixels do not affect thedisplay performance. Exemplary three-electrode pixel structure is shownin FIG. 7 a. The structure of the substrate active electrodes underneaththe membrane is seen, together with the active electrode located on themembrane (dotted line). The latter consists of three elements: twovertical conductive stripes 71 and 73, 3–4 μm wide, and a horizontalconductive stripe 72 connecting together all three stripes. Theelectrode 71 is similar to that in the two-electrode structure shown inFIG. 3. The new electrode 73 is placed somewhere in the middle of themembrane.

Unlike the previous, two-electrode, design shown in FIG. 3, there aretwo independent active electrodes on the substrate, 74 and 75,electrically isolated from each other and connected to two separateconductive lines. The left electrode 74 is similar to that in FIG. 3 andis placed underneath and parallel to the active membrane conductivestripe 71. This electrode is connected to the conductive line 76 and isalways grounded for all pixels over the display. The right electrode 75is located on the substrate underneath and parallel to the conductivestripe 73 on the membrane. The electrode 75 is connected to the row line77 in every horizontal pixel array. The row lines 77 for different pixelarrays are electrically isolated from each other.

The column line connects the bottom electrodes 71 of the membranes ineach vertical pixel array using pillar pads 78 and 78′. Thus, there aretwo pairs of active electrostatic electrodes in the pixel structure,71–74 and 73–75, having similar geometry and capacitance. However, theirelectrostatic torques are very different: the shoulder of the rightlever (L/2) is much greater than that of the left lever (d). Therefore,under application of the holding voltage V_(on) to both left electrodes71–74 and to the right electrodes 73–75, dominating electrostatic forceof the right electrodes will hold the membrane in its original,“off”-state. Such a three-electrode pixel structure allows realizationof a simple and reliable passive driving architecture for performingfunction “pixel select” and “pixel hold”.

FIG. 7 b. illustrates an example of the display matrix withthree-electrode pixel structure. Each pixel, shown as a square box, hasthree output contacts: the electrode on the substrate 75 of FIG. 7 a isconnected to the row line; the electrode 74 on the substrate ispermanently connected through the line 76 to the ground, and themembrane electrodes 71 and 73 are connected together with the connectingstripe 72 and attached to the column line. Nine pixels are presented asa part of the display matrix. All of them originally have the columnvoltage V_(on), and all rows are grounded thereby forming the “off”state of the screen.

Pixel 5 is activated into the “on”-state by applying correspondingcolumn and row pulses, while all others remain in the “off”-state. Toaccomplish transfer of the pixel 5 into the “on”-state, the pulse ofamplitude V_(snap) is applied to the middle column.

Simultaneously, the row enabling pulse of amplitude V_(snap)+V_(on), isapplied to the middle row, thereby nullifying the resultant voltage dropacross the electrodes 73–75 of the pixel 5. In this case, the onlyremaining electrostatic force is the force between the electrodes 71 and74: the column voltage pulse V_(snap) is applied to the electrode 71,while the counter electrode 74 is always kept at zero potential. Theresultant voltage V_(snap)+V_(on) will transfer the pixel 5 into the“on”-state. Other pixels in this row, i.e. pixels 4 and 6, having theinitial constant column voltage V_(on), will remain in their original“off”-state, since the row enabling pulse V_(snap)+V_(on) applied to theelectrode 75 increases the force holding the pixels in this state.

After transferring the pixel 5 into the “on”-state and removal of thecolumn and row pulses, the pixel 5 will stay in this state under theholding voltage V_(on), since after rotation of the membrane into itsupright position the electrostatic force between the electrodes 73 and75 practically disappears because of a large distance between theseelectrodes, while the force between the electrodes 71 and 74 reaches itsmaximum. One can roughly estimate the ratio of the force values betweenthe electrodes 73 and 75 in the “off”- and “on”-states as ˜(L/1.4d)²≈1.25·10³ for L=200 μm and d=4 μm.

It is important that in the presented three-electrode pixel design allother pixels in the display will not be affected by the cross talkdiscussed earlier in reference to FIG. 6. Indeed, in the “off”-state,any additional voltages applied to the pixel through the matrix loops(see FIG. 6) will increase the force between the electrodes 73–75already holding the pixel in the “off”-state. On the other hand, in the“on”-state, the force between the electrodes 73–75 becomes negligible,so that any additional voltages at these electrodes cannot change thisstate.

As in the case of two-electrode display, different coating options formaking transmissive and reflective display versions exist forthree-electrode pixel structure. These options are discussed inreference to FIG. 2, and there is no need to repeat them here.

Thus, the proposed two- and three-electrode, bi-stable pixel structures,according to the present invention, provide functions “pixel select” and“pixel hold” using simple passive matrix architecture, in contrast withconventional and rather expensive active matrix driving scheme, where anetwork of transistors and capacitances is used to execute thesefunctions. Furthermore, “pixel hold” operation, which is by far thelongest procedure in the book reading process, in the proposed displaydoes not consume power, which makes the display very attractive fore-book applications. The “pixel-select” operation requires only smallhigh frequency power component, since there is no DC current across thedisplay.

The above analysis shows that three-electrode pixel structure is apreferred embodiment for the proposed display due to more reliable pixeladdressing and switching.

FIG. 8 shows key steps for the display fabrication. The first step isdeposition of the metal layer on the substrate and photolithographypatterning and etching to fabricate the active electrode structure, FIG.8 a. A thin layer of insulator is deposited on the substrate to insulatethe metal structure from the active electrode on the membrane, when themembrane is moved to its upright position. Then, the pillars of 3–4 μmhigh are grown on the substrate. For this purpose, the first sacrificiallayer, 3–4 μm-thick, is deposited, patterned and etched to form thepillars, FIG. 8 b. Then, substrate blackening is performed if thereflection device mode is chosen. It is followed by deposition of asecond sacrificial layer, such as polyimid, having thickness equal tothe pillar height, FIG. 8 c.

Electrochemical polishing is then needed to provide a flat surface forfurther processing. Fabrication of the membrane structure starts fromdeposition of the metal on the second sacrificial layer,photolithography patterning end metal etching to form the membraneactive electrode, FIG. 8 d. It is followed by deposition of theinsulator layer, preferably, SiO₂, FIG. 8 e, and deposition of the topreflective metal layer, preferably Al, if reflective mode display isconsidered, FIG. 8 f. The alternative process would be deposition of awhite paint, or a black powder, depending on the chosen mode of thedisplay operation. It is followed by patterning and etching of both thetop layer and SiO₂ film to form the pixel structure. Finally, thesacrificial layer is removed by plasma etching to provide an air gapbetween the membrane and the substrate, FIG. 8 g. To avoid etching ofthe pillars by plasma etching, the first sacrificial layer must beresistive to the plasma etching process chosen for removal of the secondsacrificial layer. All these processes are well within capabilities ofmodern, plane MEMS technology. Only four photolithography masks areneeded for the device processing. This makes the device manufacturingsimple and inexpensive.

In the above description, each pixel contains only one membraneactivated by the electrostatic force. One can however realize anotherdesign wherein each pixel has two or more membranes. Every membranecovers half or one third of the pixel area if two or three membranes perpixel are respectively used. Each small membrane can be activated eitherseparately or simultaneously with another membrane (or membranes ) inthe pixel, thereby providing a capability for gray levels. As in theprevious, single membrane, design, two- or three-electrode structuresfor individual activation of the small

The invention claimed is:
 1. A two-electrode optical electromechanicalshutter comprising: a substrate; a membrane originally resting on fourpillars above said substrate; said membrane being composed of thefollowing elements: a first conducting electrode facing said substrateand attached to two pillars of said four pillars through torsion hingesmade from the same material as said conducting electrode; said firstconducting electrode is placed at the membrane edge in the shape of astripe parallel to said membrane edge and has the width equal or lessthan the depth of the air gap existing between said substrate and saidmembrane, so that said membrane is capable of rotating around saidtorsion hinges and reaching position normal to the substrate plane; alayer of insulator above said conducting electrode; a top layer on saidinsulator. a second conducting electrode placed on said substrateunderneath said first conducting electrode on said membrane and made asa stripe parallel to said first conducting electrode, so that underapplication of appropriate potential difference between said first andsecond conducting electrodes said membrane can move to the positionnormal to said substrate plane; output conductive lines connected tosaid first and second conductive electrodes for application of a voltageto the shutter.
 2. The shutter of claim 1 wherein said torsion hingesare made within the membrane area by cutting notches through saidmembrane to form narrow and flexible belts extending to said two pillarsof said four pillars.
 3. The shutter of claim 1, wherein the resilientproperties of said torsion hinges are properly adjusted to produceeffect of shutter bi-stability, when the potential difference betweensaid first and second conductive electrodes needed to rotate saidmembrane into its final position normal to the substrate plane issignificantly larger than the potential difference between said firstand second electrodes needed to maintain said membrane in said finalposition.
 4. The shutter of claim 3, wherein said substrate is made fromglass.
 5. A display comprising a two-dimensional array of the shuttersof claim 3 which form a matrix of pixels, wherein said first conductiveelectrodes of all shutters in every vertical array are connected to acolumn conductive line and said second conducive electrodes of allshutters in every horizontal array are connected to a row conductiveline, so that every pixel in the display is associated with a particularpair of said column and row lines to selectively apply voltage to thepixel located at the intersection of these lines.
 6. The display ofclaim 5, wherein the operation of the display activation to produce animage is followed by holding all pixels in the display at a constantvoltage, which is lower than the voltage needed to transfer pixel intothe “on”-state.
 7. The display of claim 6, in which for reflecting modeof the display operation the substrate is blackened and absorbs light,while said top layer on said insulator is a white color powder toenhance the contrast ratio.
 8. The display of claim 6, in which forreflecting mode of the display operation the substrate is blackened andabsorbs light, while said top layer on said insulator is made as a lightreflective metal to enhance the contrast ratio.
 9. The display of claim6, wherein for reflective mode of the display operation the substrate iscoated with a white color film, while said top layer on said insulatoris a black powder to absorb light and increase the contrast ratio. 10.The display of claim 6, wherein for transmissive mode of the displayoperation the substrate is made from a transparent material to passlight in the “on”-state, and said top layer on said insulator is a blackpowder, to absorb light and thus increase the contrast ratio.
 11. Themethod of fabrication of the display of claim 6 comprising the followingsteps: providing the substrate; metal deposition, photolithographypatterning and metal etching to form said second conductive electrodeand said row lines; deposition of the first sacrificial layer,photolithography patterning and etching to fabricate said four pillars;deposition of the second sacrificial layer; electromechanical etching toproduce flat top surface with thickness of said second sacrificial layerequal to height of said four pillars; metal deposition, photolithographypatterning and etching to form said first conducting layer and saidcolumn lines; deposition of said layer of insulator; deposition of saidtop layer on said insulator; photolithography patterning and etching ofsaid top layer on said insulator and said layer of insulator to formsaid membranes; plasma etching of said second sacrificial layer to formpixels.
 12. A three-electrode electromechanical shutter comprising: asubstrate; a membrane originally resting on four pillars above saidsubstrate; said membrane being composed of the following elements: afirst conducting electrode facing said substrate and attached to twopillars of said four pillars through torsion hinges made from the samematerial as said conducting electrode; said first conducting electrodeconsists of three conducting pieces: the first piece is placed at themembrane edge in the shape of a stripe parallel to said membrane edgeand has the width equal or less than the width of the air gap existingbetween said substrate and said membrane, so that said membrane iscapable of rotating around said torsion hinges and reaching positionnormal to the substrate plane; the second piece in the shape of a stripeof the same width as said first piece is placed in the middle of saidmembrane parallel to said first piece; the third piece in the shape of astripe is placed normally to said first and second pieces and connectssaid first and second pieces into a single said first conductingelectrode; a layer of insulator above said conducting electrode; a toplayer on said insulator; a second conducting electrode placed on saidsubstrate underneath said first piece of said first conductive electrodeon said membrane and made as a stripe parallel to said first piece; athird conducting electrode placed on said substrate underneath andparallel to said second piece of said first conducting electrode; outputconductive lines connected to said first, second and third conductiveelectrodes for application of voltages to the shutter.
 13. The shutterof claim 12 wherein said torsion hinges are made within the membranearea by cutting notches through said membrane to form narrow andflexible belts extending to said two pillars of said four pillars. 14.The shutter of claim 12, wherein the resilient properties of saidtorsion hinges are properly adjusted to produce effect of shutterbi-stability, when the potential difference between said first andsecond conductive electrodes needed to rotate said membrane into itsfinal position normal to the substrate plane is significantly largerthan the potential difference between said first and second electrodesneeded to maintain said membrane in said final position, while nopotential difference is applied between said first and third conductingelectrodes.
 15. The shutter of claim 12, wherein said substrate is madefrom glass.
 16. The shutter of claim 12, wherein said first and secondconductive electrodes are made from metal.
 17. The shutter of claim 12,in which for the reflecting mode of the shutter operation the substrateis blackened and absorbs light, while said top layer on said insulatoris a white color powder or paint to enhance the contrast ratio.
 18. Theshutter of claim 12, in which for reflecting mode of the shutteroperation substrate is blackened and absorbs light, while said top layeron said insulator is made as a light reflective metal to enhance thecontrast ratio.
 19. The shutter of claim 12, wherein for reflective modeof the display operation the substrate is coated with a whitedielectric, while said top layer on said insulator is a black powder.20. The shutter of claim 12, wherein for the transmissive mode of theshutter operation the substrate is made from a transparent glass to passthe light in the “on”-state, while said top layer on said insulator is ablack powder.
 21. A display comprising a two-dimensional array of theshutters of claim 13 which forms a matrix of pixels, wherein said firstconductive electrodes of all shutters in every vertical array areconnected to a column conductive line and said third conduciveelectrodes of all shutters in every horizontal array are connected to arow conductive line, while all said second conductive electrodes on saidsubstrate are grounded.
 22. The display of claim 21, wherein theoperation of the display activation to produce an image on the screen isfollowed by holding all pixels in the display at a constant voltage,which is lower than the voltage needed to transfer pixel into the“on”-state.
 23. The display of claim 22, in which for reflecting mode ofthe display operation the substrate is blackened and absorbs light,while said top layer on said insulator is a white color powder toenhance the contrast ratio.
 24. The display of claim 22, in which forreflecting mode of the display operation the substrate is blackened andabsorbs light, while said top layer on said insulator is made as a lightreflective metal to enhance the contrast ratio.
 25. The display of claim23, wherein for reflective mode of the display operation the substrateis coated with a white dielectric, while said top layer on saidinsulator is a black powder to absorb light and increase the contrastratio.
 26. The display of claim 23, wherein for transmissive mode of thedisplay operation the substrate is made from a transparent glass to passlight in the “on”-state, while said top layer on said insulator is ablack powder, to absorb light and thus increase the contrast ratio.